Heating device coated with a self-cleaning coating

ABSTRACT

The invention relates to a heating device ( 1 ) comprising a metallic substrate ( 2 ) whose at least one part is coated with a self-cleaning coating. The inventive coating consists of an external layer ( 4 ) contacting ambient air and comprising at least one type of oxidation catalyst selected from platinoid oxides, at least one internal layer ( 3 ) which is arranged between the metallic substrate and the external layer and comprises at least one type of oxidation catalyst selected from transition elements oxides of  1 b group. The inventive heating device can be embodied, for instance in the form of an iron soleplate consisting of a heating base ( 6 ) provided with heating elements ( 7 ) or a cooking appliance. Said metallic substrate can be covered with an intermediary enamel layer ( 5 ). A method for coating the metallic substrate of a heating device with said coating is also disclosed.

The present invention relates to heating devices or devices intended tobe heated during their use and comprising a self-cleaning coating.

Certain heating devices, such as for example pressing iron soleplates orcooking devices, have qualities of ease of use and effectiveness,dependent inter alia on the state and nature on surface on theircoating.

Pressing iron soleplates have been able to be improved by the attentiongiven to the sliding qualities of the ironing surface, combined withqualities allowing easier spreading out of the linen. A way of obtainingthese qualities is to resort to soleplates enamelled with an enamel ofsmooth aspect, with possibly lines of extra thickness making it possibleto spread out the fabric during the displacement of the iron. Othermetal soleplates treated mechanically and/or covered or not with adeposit to facilitate sliding can also be appropriate for a satisfactoryuse.

However, with use, the soleplate can tarnish by carbonizing in a more orless diffuse way on its ironing surface, and in a more or lessincomplete way, the various organic particles collected by friction onironed fabrics.

But when the soleplate is tarnished, even in a manner that is not veryvisible, it partially loses its sliding qualities. Imperceptibly, withthe fouling, ironing becomes more difficult. In addition, the user isreluctant to use a tarnished iron, fearing that it can deteriorate theirlinen.

Pressing iron soleplate coatings are known having a hard and resistantlayer covered, as indicated by the patent U.S. Pat. No. 4,862,609, by alayer improving the surface properties. But this patent does notindicate a solution to deal with fouling.

The walls of the cooking devices are also often covered with anenamelled layer of smooth aspect so that possible projections of greaseor food do not adhere to surface. Enamelled self-cleaning surfaces areknown, for example in ovens and cooking utensils as described forexample in patent U.S. Pat. No. 4,029,603 or patent FR2400876.

However, these coatings does not give complete satisfaction with regardto their self-cleaning properties.

There thus exists the need for a coating for a heating device likecooking devices or pressing iron soleplates, which maintains the coveredsurface clear of any contamination by organic particles, and is notfouled in normal use, in order to preserve its initial qualities.

The present invention relates to a heating device comprising a metalsupport of which at least a part is covered with a self-cleaningcoating, characterized in that the coating comprises:

-   -   a°) an external layer, in contact with the ambient air,        comprising at least one oxidation catalyst chosen among oxides        of platinoids,    -   b°) at least one internal layer, located between the metal        support and the external layer, comprising at least one        oxidation catalyst chosen among the oxides of the transition        elements of Group 1b.

The present invention also has as an aim a process for covering themetal support of a heating device with a self-cleaning coating such asabove, characterized in that it comprises the following steps:

-   -   i) one heats the surface of the metal support to be covered in        an oven at around 400° C.,    -   ii) one places the surface of the metal support to be covered        under infra-red at a temperature of 400° C. to 600° C. for a few        seconds,    -   iii) one sprays a solution of an oxidation catalyst precursor        chosen among oxides of the transition elements of Group 1b on        the surface of the metal support to be covered to obtain the        internal layer,    -   iv) one again heats the surface of the metal support to be        covered, with the internal layer, in an oven at around 400° C.,    -   v) one places the surface of the metal support to be covered,        with the internal layer, under infra-red at a temperature of        400° C. to 600° C. for a few seconds,    -   vi) one sprays a solution of an oxidation catalyst precursor        chosen among oxides of platinoids on the internal layer to        obtain the external layer,    -   vii) one reheats the surface of the metal support covered with        the internal and external layers under infra-red for a few        minutes.

Owing to the invention, one obtains a device whose self-cleaning coatingpresents a particularly excellent catalytic activity and whose adherenceto the metal support is very good.

It was in effect noted that the association of an oxidation catalystchosen among oxides of the transition elements of Group 1b in theinternal layer with an oxidation catalyst chosen among oxides ofplatinoids in the external layer increased in operation theself-cleaning activity of the coating in a synergistic way.

Owing to the invention, the organic particles in contact with theexternal layer of the coating are oxidized when the device is heated. Inaddition, the effect of synergy obtained by the particular associationof an internal layer comprising a specific oxidation catalyst and of anexternal layer comprising a specific oxidation catalyst different fromthat of the internal layer makes it possible to obtain a coatingpresenting a particularly powerful catalytic activity. Thus, the surfaceof the coating is restored very quickly.

For example, during ironing with a pressing iron, the organic particlescollected by the soleplate are oxidized. They are to some extent burnedwhen the pressing iron is hot, the possible solid residue loses anyadherence and is detached from the soleplate. The soleplate ismaintained clean.

Similarly, in a cooking device such as an oven for example, projectionsof grease present on the wall of the oven are oxidized while hot, thesolid residue is detached from the wall, which is maintained clean.

Moreover, owing to the process of the invention and in particular owingto the exposure of the surface of the metal support to be covered to theinfrared, the adherence of the coating to the metal support isparticularly good. This improved adherence makes it possible to increasethe friction resistance of the coating, this property being particularlyadvantageous in the case of a pressing iron soleplate for example.

By “heating device”, one understands within the meaning of the presentrequest, any device, article or utensil, which during its operationreaches a temperature at least equal to 45° C., and preferably at leastequal to 90° C. The device can reach this operating temperature by meanswhich are specific to it, as for example a heating base integrated intothe device and provided with heating elements, or by external means.Such devices are for example pressing iron soleplates, cooking devices,ovens, grills, kitchen utensils.

The external coating layer according to the invention comprises anoxidation catalyst chosen among oxides of platinoids. By “platinoids”,one understands, within the meaning of the present request, the elementshaving properties similar to those of platinum, and in particular, inaddition to platinum, ruthenium, rhodium, palladium, osmium and iridium.Preferably, the external layer comprises an oxidation catalyst chosenamong palladium oxides, platinum oxides and their mixtures.

In practice, such oxidation catalysts are well-known in themselves, aswell as their fabrication processes, without it being necessary todescribe in detail their methods of preparation respectively. Thus, asan example, as regards platinum as an oxidation catalyst, itscatalytically active form can be obtained by calcination ordecomposition of a chloroplatinic acid salt or any other precursor.

Of course any oxidation catalyst used according to the present inventionwill have to remain sufficiently stable at the operating temperature ofthe device, and this within the limits of the useful lifespan of thedevice.

The surface of the external layer is directly in contact with theambient air and the organic stains. By “organic stains”, one understandswithin the meaning of the present application any substance combustibleor oxidizable, completely or partially, in contact with the ambient air.As an example, one can cite any synthetic fiber residue, as used intextile articles, for example of organic polymer such as polyamide orpolyester, any organic residue of a washing product and possibly of asoftening product, any organic substance such as projections of greasesor foods.

The oxidation catalyst chosen among oxides of platinoids is distributedon and/or in the external coating layer, where it is in contact with thestains, and over whole or part of the external layer, in a continuous ordiscontinuous way.

In the case of a pressing iron soleplate which comprises or not reliefzones, the oxidation catalyst chosen among the platinoids is distributedon the external surface of the soleplate, intended to be put in contactwith the linen.

The coating can comprise, in addition to the oxidation catalyst chosenamong oxides of platinoids, any other internal support layer that iscatalytically inert with regard to oxidation. This support adherent tothe metal support and catalytically inert is preferably selected amongthe compounds of aluminum or silicon, such as for example alumina individed form or in particles, enamel, polytetrafluoroethylene and theirmixtures.

In a preferred embodiment of the invention, the support that iscatalytically inert with regard to oxidation is an enamel with lowporosity and/or roughness, on a micrometric and/or nanometric scale. Theenamel is for example a vitrified enamel. The enamel should preferablybe hard, slide easily and resist the penetration of hot steam ormoisture.

The external layer of the coating preferably has a thickness, measuredaccording to the RBS method described in Example 1 of the presentapplication, extending from 10 nanometers to 500 nanometers, andpreferably still extending from 20 nanometers to 120 nanometers.

The oxidation catalyst of the external layer being active at a coatingtemperature greater than or equal to 90° C., it cleans said coating whenthe latter is heated at least to such a temperature.

The internal layer comprises at least one oxidation catalyst chosenamong oxides of the transition elements of Group 1b, preferably selectedamong copper oxides, silver oxides and their mixtures.

In practice, such oxidation catalysts are well-known per se, as well astheir production processes, without it being necessary to describe indetail their methods of preparation respectively. As an example,concerning silver oxide as an oxidation catalyst, one can use as aprecursor silver nitrate sold commercially by the Aldrich company.

Preferably, the catalytically active internal layer has a thickness,measured according to the RBS method described in Example 1 of thepresent application, extending from 20 nanometers to 50 nanometers.

Preferably, the oxidation catalyst present in the internal layer has agood affinity with the oxidation catalyst present in the external layer.In effect, after application on the support of the internal and externallayers, the support is reheated and, during this step, the oxidationcatalyst present in the internal layer can diffuse into the externallayer and the oxidation catalyst present in the external layer candiffuse into the internal layer. In a preferred embodiment of theinvention, the external layer comprises as oxidation catalyst an oxideof palladium and the internal layer comprises as oxidation catalyst asilver oxide. In a more preferred embodiment of the invention, thesilver oxide has diffused into the external layer and the external layerthus comprises a mixture of palladium oxide and silver oxide. There wasobserved a particular synergy effect at the level of the catalyticactivity of the coating in such an embodiment of the invention.

In a preferred embodiment of the invention, the heating device is in theshape of a pressing iron soleplate comprising an ironing surface andcoating covers the ironing surface.

In another preferred embodiment of the invention, the heating device isa cooking device comprising walls likely to come in contact with organicstains and coating covers these walls.

In a first operating mode, the catalyst acts at the operatingtemperature of the device and the coating is kept clean as the device isbeing used.

In a second operating mode, at the time of a phase termed self-cleaning,before or after use of the device, the latter is adjusted to a hightemperature, equal to or higher than the highest operating temperatures,it is then left on standby during a predetermined time, during which theoxidation catalyst takes effect. The user can thus regularly maintainhis device, without awaiting a harmful soiling.

The metal support of the device according to the invention can be basedon any metal usually employed in the field of the heating devices likealuminum, steel or even titanium. This metal support can itself becovered with a protective layer as for example an enamelled layer beforebeing covered by the coating of the present invention. Thus, in apreferred embodiment of the invention, the device comprises an enamelintermediate layer located between the metal support and thecatalytically active internal layer of the coating.

The application of catalytically active internal and external layers onthe metal support, covered or not by an enamelled layer, is donepreferably by pyrolysis, by heating of the surface to be covered thenspraying on this hot surface of a solution containing a precursor of theoxidation catalyst. By “precursor”, one understands any chemical orphysicochemical form of oxidation catalyst, which is likely to lead to,or to liberate this latter by any appropriate treatment, for examplepyrolysis.

In an embodiment of the process according to the invention, the surfaceof the metal support to be covered is heated in an oven to around 400°C. then placed very briefly, for example during a few seconds, underinfra-red, until reaching a surface temperature that can go from 400° C.to 600° C. This operation softens the surface of the support and makesit possible to increase the later adherence of the coating. A solutionof the oxidation catalyst precursor chosen among the transition elementsof Group 1b is sprayed onto the surface of the metal support. On contactwith the surface, the precursor oxidizes and is fixed on the support andwater evaporates. A layer with a thickness extending from 20 to 50 Nm isdeposited. The support cools very quickly. It is heated again by in theoven to 400° C. then under infra-red to a temperature which can extendfrom 400° C. to 600° C. during a few seconds. A solution of theoxidation catalyst precursor chosen among the platinoids is sprayed overthe internal layer. A layer of a thickness extending from 20 to 50 Nm isdeposited. The support thus covered is then reheated under infra-redduring a few minutes, for example during five minutes.

One obtains a support covered with a coating whose self-cleaningproperties are particularly good.

The invention will be better understood by reading the exampleshereafter and the annexed drawings.

FIG. 1 is a cross-sectional view of a pressing iron soleplate accordingto the invention,

FIG. 2 is a cross-sectional view of a soleplate of pressing ironaccording to the invention comprising an enamelled protective coating.

Referring to FIG. 1, there is shown in cross section a heating device 1in the shape of a pressing iron soleplate comprising a metal support 2covered with an internal layer 3 and an external layer 4. The soleplatealso comprises a heating base 6 provided with heating elements 7.Support 2 and base 6 are assembled by mechanical means or by gluing.Internal layer 3 comprises an oxidation catalyst chosen among oxides ofthe transition elements of Group 1b and external layer 4 comprises anoxidation catalyst chosen among oxides of platinoids.

Referring to FIG. 2, there is shown in cross section a pressing ironsoleplate comprising a metal support 2 covered with an intermediatelayer 5, an internal layer 3 and an external layer 4. The soleplate alsocomprises a heating base 6 provided with heating elements 7, glued ontosupport 2. Internal layer 3 comprises an oxidation catalyst chosen amongoxides of the transition elements of Group 1b and external layer 4comprises an oxidation catalyst chosen among oxides of platinoids.Protective layer 5 is of enamel.

EXAMPLE 1

A clean pressing iron soleplate of enamelled aluminum is placed on analuminum support of approximately 2 cm to store heat as well aspossible. The unit is heated to 400° C. in an oven. The soleplate, withthe support, is placed during a few seconds under infra-red untilreaching a surface temperature between 400° C. and 600° C. Silvernitrate, sold by the Aldrich company, is placed in solution in water at4 g/l and is sprayed by means of an air gun onto the soleplate. A layerof around 40 to 50 Nm, measured according to RBS method, is deposited.The RBS (Rutherford Backscattering Spectroscopy) method is a techniqueof analysis based on the elastic interaction between a beam of ⁴He²⁺ions and the component particles of the sample. The high energy (2 MeV)beam strikes the sample, the retrodiffused ions are detected at an angleteta. The spectrum thus acquired represents the intensity of the ionsdetected according to their energy and makes it possible to determinethe thickness of the layer. This method is described in W. K. Chu and G.Langouche, MRS Bulletin, January 1993, p 32.

After the application of this internal layer, the soleplate is heated inthe oven to 400° C. then placed again during a few seconds underinfra-red to a temperature between 400° C. and 600° C. An aqueoussolution of palladium nitrate stabilized by nitric acid, sold by theMetalor company, is sprayed by means of a gun pneumatically on thesoleplate. A layer of around 40 to 50 Nm, measured according to RBSmethod described above, is deposited.

After the application of this external layer, the unit is reheated underinfra-red to 500° C. during three minutes.

One obtains a pressing iron soleplate whose self-cleaning coatingadheres particularly well to the soleplate and has a very good catalyticactivity and preserves its sliding qualities.

1) Heating device (1) comprising a metal support (2) of which at least apart is covered with a self-cleaning coating, characterized in that thecoating comprises: a°) an external layer (4), in contact with theambient air, comprising at least one oxidation catalyst chosen among theoxides of platinoids, b°) at least one internal layer (3), locatedbetween the metal support (2) and the external layer (4), comprising atleast one oxidation catalyst chosen among oxides of the transitionelements of Group 1b. 2) Device according to claim 1, characterized inthat the oxidation catalyst of the external layer (4) is selected amongpalladium oxides, platinum oxides and their mixtures. 3) Deviceaccording to claim 1, characterized in that the oxidation catalyst ofthe internal layer (3) is selected among copper oxides, silver oxidesand their mixtures. 4) Device according to claim 1, characterized inthat the external layer (4) comprises as oxidation catalyst a palladiumoxide and the internal layer (3) comprises as oxidation catalyst asilver oxide. 5) Device according to claim 4, characterized in that theexternal layer comprises a mixture of palladium oxide and silver oxide.6) Device according to claim 1, characterized in that the thickness ofthe external layer (4), measured according to the RBS method, extendsfrom 10 to 500 nanometers, and more preferably extends from 20nanometers to 120 nanometers. 7) Device according to claim 1,characterized in that the thickness of the internal layer (3), measuredaccording to the RBS method, extends from 20 nanometers to 50nanometers. 8) Device according to claim 1, characterized in that itfurther comprises an intermediate layer (5) located between the metalsupport (2) and the internal layer (3) of the coating constituting asupport that is catalytically inert with regard to oxidation selectedamong aluminum alloys, enamel, polytetrafluoroethylene and theirmixtures. 9) Device according to claim 8, characterized in that theintermediate layer (5) located between the metal support (2) and theinternal layer (3) of the coating is of enamel. 10) Device according toclaim 1, characterized in that said device is in the shape of a pressingiron soleplate comprising an ironing surface and that the coating coversthe ironing surface. 11) Device according to claim 1, characterized inthat said device is in the shape of a cooking device comprising wallslikely to come in contact with organic stains and the coating coversthese walls. 12) Process for producing the heating device (1) of claim 1said process comprising the following steps: i) heating the surface ofthe metal support to be covered in an oven at around 400° C., ii)placing the surface of the metal support to be covered under infra-redat a temperature of 400° C. to 600° C. for a few seconds, iii) sprayinga solution of an oxidation catalyst precursor chosen among oxides of thetransition elements of Group 1b on the surface of the metal support tobe covered to obtain the internal layer (3), iv) again heating thesurface of the metal support to be covered, with the internal layer, inan oven at around 400° C., v) placing the surface of the metal supportto be covered, with the internal layer, under infra-red at a temperatureof 400° C. to 600° C. for a few seconds, vi) spraying a solution of anoxidation catalyst precursor chosen among oxides of platinoids on theinternal layer to obtain the external layer (4), and vii) reheating thesurface of the metal support covered with the internal and externallayers under infra-red for a few minutes.